Planets This Week

This week, the moon will rise very late and wane in phase to last quarter, leaving evening skies the world over darker and darker. Here’s what the planets are up to this week.

In the eastern late-evening sky on Tuesday, September 17, the waning gibbous moon will be positioned about a palm's width below the blue-green planet Uranus. Blue-green Uranus will be rising in the east just before 9 pm local time this week; and it will remain visible all night long. Uranus is sitting below (or to the celestial south of) the stars of Aries (the Ram) and is just a palm’s width above the head of Cetus (the Whale). At magnitude 5.8, Uranus is actually bright enough to see in binoculars and small telescopes, under dark skies. You can use the three modest stars that form the top of the head of the whale (or sea-monster in some tales) to locate Uranus this autumn - that’s because the distant planet moves so slowly in its orbit.

Yellow-tinted Saturn is prominent in the southern evening sky, too - but it is much less bright than Jupiter. The ringed planet will be visible from dusk until about 1 am local time. Saturn’s position in the sky is just to the upper left (or celestial east) of the stars that form the teapot-shaped constellation of Sagittarius (the Archer). To find Saturn, look about 3 fist diameters to the left (celestial east) of Jupiter. The Milky Way is between them.

On Wednesday, September 18, Earth's faster orbit will cause Saturn to end a westward retrograde loop with respect to the distant stars and resume its regular eastward motion.

The earlier-arriving sunsets of September are allowing us to view spectacularly bright Jupiter for a while. As the sky begins to darken this week, look for the giant planet Jupiter sitting less than a third of the way up the southwestern sky. Jupiter has been spending this year below Ophiuchus (the Serpent-Bearer) and above Scorpius (the Scorpion). Antares, the very bright star that is sitting less than a fist’s diameter to the lower right (or celestial southwest) of Jupiter, marks the heart of the scorpion.

Mars is now pulling away from the sun’s glare and will become visible in the eastern pre-dawn sky later this month. Unfortunately, the red planet is on the far side of the sun from us and will remain small and faint until 2020 begins.

Meanwhile, Mercury and Venus are also sitting quite close to the sun. Both of those planets are in the process of swinging away from the glare of sunset. This week, you might be able to see Mercury and Venus sitting very low in the west for about 20 minutes after the sun disappears. The very shallow angle that the evening ecliptic makes with the western horizon will prevent people in the Northern Hemisphere from seeing them very easily at this time. But observers near the equator and in the Southern Hemisphere will see them well in the next couple of weeks.

Keep looking up and enjoy the sky with Star Walk 2!